1. Field of the Invention
The present invention relates to a gradient coil system for a magnetic resonance apparatus with an area provided to receive an examination subject.
2. Description of the Prior Art
Magnetic resonance is a known technology for acquiring images of the inside of a body. In a magnetic resonance apparatus, rapidly switched gradient fields are superimposed on a static basic magnetic field that is generated by a basic field magnetic system. Furthermore, the magnetic resonance apparatus has a high-frequency system which irradiates high-frequency signals into the examination subject for triggering magnetic resonance signals and which detects the generated magnetic resonance signals on the basis of which magnetic resonance images are produced.
The gradient system includes a gradient coil system having gradient coils and controlled gradient amplifiers. One of the gradient coils generates a gradient field for a specific spatial direction. This gradient field, in the ideal case, has only a main field component, which is collinear to the basic magnetic field, at least within an imaging volume. The main field component has a prescribable main gradient which, at any arbitrary point in time, by approximation, has the same magnitude at all locations manner at least within the imaging volume. Since the gradient field is a chronologically variable magnetic field, the aforementioned is true for any point in time, however, the intensity of the main gradient is variable from one point in time to another point in time. The design of the gradient coil normally prescribes the direction of the main gradient.
Due to Maxwell""s fundamental equations, gradient coils, contrary to the desired ideal case, cannot be fashioned which generate only the aforementioned main component over the imaging volume. At least one accompanying field component, which is perpendicularly directed to the main field component, is unavoidably associated with the main field component.
Appropriate currents must be adjusted in the gradient coil in order to generate the gradient field. The amplitudes of the required currents are several 100 A. The current increase and decrease rates (xe2x80x9cslew ratexe2x80x9d) are several 100 kA/s. For the current supply, the gradient coil is connected to a controlled gradient amplifier.
As a result of the switching of the gradient fields, stimulations can be triggered in a living examination subject during the pickup of magnetic resonance images. The gradient fields acting on the examination subject are characterized by a chronologically varying, magnetic flux density generating eddy currents and induction currents in the examination subject. The intensity of the aforementioned electrical currents, among other things, depends on the cross-sectional area in which the gradient field acts and on the chronological behavior of the gradient field. The aforementioned currents traverse regions of the examination subject with different electrical conductivity and thereby effect corresponding electrical voltages. If the voltage exceeds a specific threshold, stimulations of the examination subject are triggered. For example, it is known from German OS 42 25 592 that the highest current values or voltage values, given switched gradient fields, are induced at the edge or outside of the imaging volume where the field boost of the magnetic flux density of the gradient field is at a maximum, so that the danger of stimulations is the highest there.
For preventing such stimulations, it is known from German OS 42 25 592 to cover stimulation-sensitive regions, outside of the imaging volume, with a closed conductor loop. As a result, the currents induced in the covered region are reduced. The aforementioned cover, however, is only possible outside of the imaging volume, but not in edge regions of the imaging volume, since the linearity of the gradient fields in the imaging volume and the homogeneity of the basic magnetic field are otherwise impaired, the linearity of the gradient fields in the imaging volume being crucial for the image quality. Another disadvantage is that the position of the conductor loops normally must also be adapted when a region of the examination subject to be imaged is modified.
German PS 195 27 020, in a hollow cylindrically shaped gradient coil system for a transversal gradient coil, describes a combination of a segment gradient coil and a gradient coil that is composed of saddle shaped sub-coils. The combination is to maintain the advantages of both types of gradient coils and their disadvantages are to be reduced at the same time. An intense accompanying field component, which is more intense than the usable main field component, is a disadvantage of the gradient coil that is composed of saddle shaped sub-coils. Among other things, the combination is intended to significantly reduce the accompanying field component and therefore the danger of stimulations as a result of rapidly switched gradients.
An object of the invention is to provide an improved gradient coil system with which, among other things, high intensities of a rapidly switched gradient field can be obtained without stimulating a living examination subject.
The object is inventively achieved by a gradient coil system for a magnetic resonance apparatus having an area provided for an examination subject, wherein at least one gradient coil with a conductor arrangement is provided for generating a magnetic gradient field having a main field component that is collinear to a basic magnetic field and at least one accompanying field component that is perpendicular to the main field component, and wherein at least one further conductor arrangement which is provided for feeding electrical current and is fashioned and arranged for generating a non-homogeneous magnetic field such that the main field component is approximately unaltered at least in the area such that the accompanying field component is reduced.
Therefore, the undesired accompanying component can be compensated at least in the area in which the living subject to be examined by magnetic resonance technology is situated. Therefore, the gradient field permeating the examination subject can be reduced to the magnetic resonance image-effective main field component, so that a stimulation probability of the examination subject can be reduced or an intensity of the gradient field that can be used without danger can be increased. In contrast to conventional shielding coils that attenuate the main field component and accompanying field component in the area approximately to the same extent, the further conductor arrangement is designed such that the accompanying field component is simultaneously attenuated to a much greater extent given an attenuation of the main field component, so that the previously described effect stimulations are less likely.